Studies are in progress to obtain information regarding the adaptations that occur in response to endurance exercise-training and the mechanisms by which these adaptations increase work capacity and endurance. We are obtaining information regarding the mechanisms responsible for the slower rate of carbohydrate utilization and lactate production in exercise-adapted, as compared to untrained, muscle at the same work rate. We are also studying the time course of the development and of the regression of the skeletal muscle and cardiac adaptations that occur in response to exercise, and the mechanisms underlying the development of these adaptations. The effects of training on the capillary muscle fiber ratio are being investigated, and studies are planned to determine the role of increased blood supply to the working muscles in the increase in work capacity induced by training. BIBLIOGRAPHIC REFERENCES: Booth, F.W. and Holloszy, J.O. (1977) Cytochrome c turnover in rat skeletal muscles. J. Biol. Chem. 252, 416-419. Holloszy, J.O., Rennie, M. J., Hickson, R.C., Conlee, R.K. and Hagberg, J.M. Physiological consequences of the biochemical adaptations to endurance exercise. Ann. N.Y. Acad. Sci. 1977, in press.